Gravitation at a micron and mixing of quarks

Abstract

Galilei Galileo would be somewhat surprised. When neutrons become ultra-cold, his famous fall experiment shows quantum aspects of the subtle gravity force in the sense that neutrons do not fall as larger objects do. In free-fall experiment they don't fall continuously. We find them on particular levels, when they come close to a reflecting mirror for neutrons. Of course, such bound states with discrete energy levels are expected when the gravitational potential is larger than the energy of the particle. Here, the quantum states have pico-eV energy, value that is smaller by many orders of magnitude compared with an electromagnetically bound electron in hydrogen atom, opening the way to a new technique for gravity experiments and measurements of fundamental properties. New motivations for gravity experiments come from frameworks where the fundamental Planck scale (the scale about 104s after the big bang where gravity becomes comparable in strength to the other interactions) is taken to the weak scale, the energy scale of the Standard Model at s after the big bang. Considering this very early stage of our universe, we have the strong feeling, that a Standard Model description is incomplete, and many new observables pointing to physics beyond the Srandard Model emerge from superstring theory, supersymmetry or other Grand Unified Theories GUT). For example, in theories with submillimeter dimensions, gauge fields can mediate repulsive gravity-like forces -1 O'O times stronger than gravity in submillimeter distances. Furthermore, the quark-mixing CabibboKobayashi-Maskawa (CKM) matrix remains unexplained in the Standard Model as well as CP-violation, which might explain the baryon-antibaryon asymmetry of the universe. Some observables of these theories require neutron physics, for others the neutron provides one of several possible ingredients.